(a) Field of the Invention
This invention relates to apparatus for measuring the angular deviation of the end of a drill string, and more particularly to apparatus of the mud pressure pulse signalling type.
(b) Description of the Prior Art
Patents and publications showing the general state of the prior art include: U.S. Pat. Nos. 2,329,732; 2,435,934; 2,762,132; 2,824,380; 3,431,654 and Society of Petroleum Engineers of AIME Paper Number SPE 765. The most pertinent prior art patents are U.S. Pat. Nos. 3,176,407 and 3,466,755, which teach apparatus which includes a pendulum within a housing placed in the drill string. The pendulum is suspended from a rod that is axially movable in the housing downwardly in response to mud pressure and upwardly in response to the cessation of mud flow. The upward movement of the rod is limited by a set of concentric stop rings of decreasing ascending diameter adapted to engage the pendulum, dependent upon the deviation of the pendulum from the axis of the string. A coding system is provided whereby relatively short movements of the pendulum and rod are translated into relatively long movements of a knob headed shaft extending radially outwardly from the housing. In addition to magnifying the length of movement of the pendulum and rod, the coding system inverts the movement whereby a minimum movement of the pendulum and rod gives rise to a maximum movement of the shaft and a maximum movement of the pendulum and rod gives rise to a minimum movement of the shaft. The inversion causes the shaft movement to be directly proportional to the deviation measured. The measure of the deviation is signaled to the surface by pressure pulses created by means of the interaction of the knob with a plurality of pulse rings in the string above the housing. The number of pulses detected at the surface is directly proportional to the angular deviation of the string. With pressure pulse signalling type deviation measuring devices, deviation may be measured each time a length of pipe is added to the string, and thereby allow substantially continuous monitoring of deviation.
Because of length and diameter constraints, the devices of the prior art are limited to measuring angles in a range from 0 to approximately 10 degrees. However, in directional drilling programs it is necessary to measure accurately angles much in excess of 10 degrees. Presently, such large angles are measured by various multi-shot survey instruments. However, the use of such instruments is costly in that considerable valuable rig time is expended in running such instruments into the hole. Additionally, multi-shot surveys are not well suited for substantially continuous monitoring of hole deviation, as would be desirable in directional drilling programs, and as is possible with deviation measuring apparatus of the pressure pulse signalling type.